Nickel-base super alloys consisting essentially of the following chemical compositions in weight percentage are conventionally known as alloys excellent in high-temperature corrosion resistance in the open air and other strongly oxidizing atmospheres:
(1) Carbon -- 0.04 to 0.25%, PA1 Chromium -- 10.0 to 25.0%,
At least one element selected from the group consisting of 0.1 to 30.0% iron, 0.1 to 10.0% tungsten, 0.1 to 10.0% molybdenum and 0.05 to 30.0% cobalt -- 50.0% of the maximum, and
Nickel and inevitable impurities: balance;
(2) The constituents described in (1) above, and additionally,
Boron -- 0.001 to 0.05%,
Zirconium -- 0.01 to 0.1%, and
At least one element selected from the group consisting of 0.001 to 0.02% magnesium, 0.001 to 0.05% calcium and 0.001 to 0.02% rare earth elements;
(3) Carbon -- 0.04 to 0.25%,
Chromium -- 10.0 to 25.0%,
Tungsten -- 10.0 to 25.0%,
Molybdenum -- 0.1 to 10.0%,
(Tungsten) + 2(Molybdenum) being 25.0% at the maximum,
Boron -- 0.001 to 0.05%,
Zirconium -- 0.01 to 0.1%,
At least one element selected from the group consisting of 0.001 to 0.02% magnesium, 0.001 to 0.05% calcium and 0.001 to 0.02% rare earth elements; and
Nickel and inevitable impurities -- balance.
In the conventional nickel-base super alloys having the aforementioned chemical compositions, chromium serves to improve the oxidation resistance of the alloys at high temperatures irrespective of the the oxidizing potential of atmosphere. These alloys therefore contain at least 10.0% chromium to achieve a desired oxidation resistance at high temperatures. With a chromium content of over 25.0%, however, the mechanical strength and the workability of the alloys are degraded. An upper limit of 25.0% is therefore established. Also, carbon serves to strengthen the alloy base and to stabilize the metallographical structure. A carbon content of over 0.25% makes it difficult to conduct a plastic working of the alloy, whereas a carbon content of under 0.04% cannot give a desired effect. Appropriate carbon contents are consequently limited to the range from 0.04 to 0.25%. Moreover, at least one element from among iron, tungsten, molybdenum and cobalt is included to intensify the solid-solution with a view to improving the mechanical properties and the workability of Ni-Cr alloys, and the contents are limited to the values specified above for obtaining desired effects. Boron, zirconium and at least one element from among magnesium, calcium and rare earth elements, with contents specified as above, are included as required, to improve the high-temperature creep property and strengthen grain boundaries of the alloy.
The conventional nickel-base super alloys mentioned above exhibit excellent corrosion resistance at high temperatures in a strongly oxidizing atmosphere such as the air. However, these super alloys, when used in an atmosphere of a low oxidizing potential, for example under vacuum or in high-temperature inert gas as in a high-temperature gas-cooled reactor with helium as the cooling medium, which has recently appeared, do not exhibit sufficient corrosion resistance at high temperatures and cannot withstand corrosion at high temperatures.
The reasons are as follows: when the aforementioned conventional nickel-base super alloys are placed in a strongly oxidizing atmosphere, the solid-solution-intensifying elements contained therein are almost totally oxidized to form such spinel oxides as NiCr.sub.2 O.sub.4 and FeCr.sub.2 O.sub.4. These spinel oxides, which are dense and adhere well to the alloy surface after once formed on the alloy surface, serve to inhibit the oxidation of the alloys thereafter. This is why these conventional super alloys exhibit excellent corrosion resistance at high temperatures in a strongly oxidizing atmosphere.
On the other hand, an atmosphere, even of an inert gas such as helium or argon or even under vacuum, practically contains at least one trace impurity from among oxygen, nitrogen, carbon monoxide, moisture, hydrogen and inorganic hydro-carbon. Said atmosphere of an inert gas or under vacuum has therefore often a low oxidizing potential (i.e., a weakly oxidizing atmosphere), and an alloy is not always free from corrosion by oxidation in such an atmosphere. In a strongly oxidizing atmosphere such as the air, a spinel oxide film consisting of composite compounds mainly comprising chromium is immediately formed and this film serves to prevent oxidation of an alloy thereafter. This spinel oxide film is hardly formed in the above-mentioned atmosphere of a low oxidizing potential. As a result, when the conventional nickel-base super alloy is exposed to the above-mentioned atmosphere of a low oxidizing potential, an oxide film slightly formed on its surface is easily spalled off, or selective oxidation of grain boundaries and internal oxidation proceed. The conventional super alloys cannot therefore sufficiently withstand corrosion at high temperatures.
In view of the foregoing, there has been a demand for an alloy showing excellent corrosion resistance at high temperatures in an atmosphere of a low oxidizing potential, but an alloy having such a property has not as yet been proposed.